The wireless portable Internet is a next generation communication system for further supporting mobility, in addition to a short range data communication system which uses stationary access points (APs) such as the conventional wireless local area network (LAN). Various standards have been proposed for the wireless portable Internet, and the international standardization on the portable Internet is in progress by the IEEE 802.16.
The wireless LAN system such as the conventional IEEE 802.11 provides a data communication system which allows short-range radio communication with reference to stationary access points, which provides no mobility of the subscriber station (SS) but which supports wireless LAN data communication in a local area other than wired LAN data communication.
Meanwhile, a new wireless portable Internet system currently progressed by the IEEE 802.16 working group is designed to support the mobility to the subscriber station and thus provide a seamless data communication service thereto when the subscriber station moves from one cell to another cell.
The mobile communication systems including the above-described wireless portable Internet system have been developed for communication systems which support speech services and high-speed packet data services.
Reported information on the radio channel quality of a link to a subscriber station on the move is very important since the information is used to determine an appropriate adaptive modulation and channel coding (AMC) level for the corresponding link to the subscriber station in the system for supporting high-speed mobility. Since the reported information on the radio channel quality is found to be erroneous, a resource allocated to the link to the subscriber station may be wasted, it is accordingly very important to provide reliable channel quality information (CQI) to a scheduler of the base station.
In order to collect information on the channel quality, the base station selects a predetermined subscriber station for each slot from among a plurality of subscriber stations, transmits packet data thereto, and receives channel quality information on a forward channel from the selected subscriber station to determine transmission parameters such as data rates, channel coding rates, and modulation orders.
In order that the subscriber station reports channel quality information to the base station, when the base station transmits a CQI report message to a plurality of subscriber stations, each subscriber station requests bandwidth for reporting channel quality information. When the bandwidth is allocated, each subscriber station reports a channel quality measurement result to the base station in an additional message format.
FIGS. 1 to 3 show general flowcharts for measuring and reporting channel quality information in a mobile communication system. In FIGS. 1 to 3, the transverse axis stands for the time passage, BS symbolizes the base station, and SS symbolizes the subscriber station.
Referring to FIG. 1, in order to measure the radio channel quality between a base station and a subscriber station, the subscriber station receives a CQI request (REP-REQ) message from the base station and requests a bandwidth for a channel measurement report therefrom (S10 to S13), the base station allocates an uplink resource (UL-MAP) to the subscriber station (S14 and S15), and the subscriber station uses the uplink resource and transmits the channel measurement report (REP-RSP) message to the base station (S16 and S17). The REP-REQ/RSP message is a channel measurement report request/response message from among media access control (MAC) managed messages defined in IEEE 802.16.
However, a delay of a predetermined time occurs because of the request and allocation of the uplink bandwidth until the subscriber station reports the channel measurement information to the base station, since the base station allocates no uplink resource to be used for the channel measurement report in advance when requesting channel quality information from the subscriber station, thereby very probably failing to quickly process the varied channel condition and satisfy the quality of service (QoS) criteria.
FIG. 2 shows a process for the subscriber stations to competitively request a bandwidth from the base station. The respective subscriber stations competitively request a bandwidth for a channel measurement report from the base stations in steps S20 to S23, and when the request has failed, the subscriber stations attempt a competitive bandwidth request again through a backoff process in steps S24 to S27. When the attempt is found to be successful, the base station allocates an uplink resource to the corresponding subscriber station, and the subscriber station transmits a channel measurement report to the base station through the allocated uplink resource in steps S28 to S30. In this case, a delay is generated by the backoff, and the request and allocation of the uplink bandwidth.
FIG. 3 shows a process for a subscriber station to transmit a random code for a bandwidth request to the base station in the general case of requesting and reporting the channel quality information.
When the subscriber station transmits a competitive random code for a bandwidth request to the base station according to the channel quality information provided by the base station in steps S40 to S43, the base station cannot determine from the bandwidth request code whether the subscriber station will transmit bandwidth request information (i.e., an amount of data stored in a transmission buffer of uplink data) or transmit the message for the channel measurement report. Accordingly, the subscriber station may be delayed in transmitting the message for the channel measurement report to the base station even though the subscriber station has successfully transmitted the random code to the base station, and hence, the time delay is inevitable. As shown in FIG. 3, when the base station allocates a resource for a bandwidth request and the subscriber station transmits a bandwidth request message before the subscriber station transmits the channel measurement report to the base station, the base station must allocate the uplink resource in steps S44 to S49, and hence, a time delay is generated and it is difficult to guarantee the QoS because of the undesired delay.
Also, the base station must transmit a CQI request message to each subscriber station that will generate a channel measurement report. Hence, when transmitting the CQI request message to a plurality of subscriber stations in a frame, the base station respectively transmits the same unicast message (e.g., a basic connection identifier (CID) according to the IEEE 802.16 standard) to the subscriber stations, thereby increasing overheads.
However, the method for the base station to request the channel quality information from a plurality of subscriber stations in the cage of one frame may exhaust downlink resources since the base station transmits similar messages to the subscriber stations individually. Further, when the base station transmits the message to the subscriber station by using an inadequate AMC level, in detail, when the base station transmits the CQI request message thereto by using the AMC level determined based on the existing channel status even though the channel has already been degraded, some subscriber stations may fail to receive the CQI request message.
Also, overheads of messages are increased when the respective subscriber stations individually transmit a response message for the channel quality measurement result to the base station.
In addition, the mobile system does not guarantee allocation of uplink resources for transmitting the response message of the channel quality measurement result, and hence, heavy delay may be generated when the subscriber station transmits the response message to the base station. As a result, the subscriber stations may fail to transmit the on-time response message thereto, and the base station may not adaptively process the message following the mobile environment